First generation Dodge Ram 2500 with the 360 (high torque, not the same as the one in the 1500) so that woudl be about 95 to 02. That should be under 10k. You could get the Cummins for probably 10 to 15k, but get one with a stick, not an auto.
An early Superduty F250 (99 to about 02) woudl be easily gettable with the V10, might be tougher with the PSD for your price range. Should be plenty of retiring work trucks like you need. Do not get the V8, it's underpowered and gets the same mileage as the V10.
this generation Chevy (01?) with the smaller motor should work fine
They are all reasonably similar. The dodge has crappy auto-transmissions, but fine manual's. the early Ford's aren't so hot on auto transmissions behind the PSD's either. Check out edmunds.com for prices, reviews, etc
OH yes, in the past 10 years i've owned
95 Dodge Ram 2500, 5.7L (360) gas, 5 spd, regular cab, long bed 4wd
03 Ford F250 Supercab, long bed, v10 4wd, 6 spd
02 Ford F350 Crew Cab dually, long bed, 7.3L PSD, 6 spd
06 Chevy 2500HD, Crew Cab long bed, Duramax, 6spd Allison (which is relaly awesome but way out of your price range)